In underground mine environments, a body or vein of ore will often be accessed by excavating cavities or working chamber (hereinafter cavity) into the rock strata below the ore body or vein and then working towards the ore deposit from below. This technique is referred to in the mining field as “overhand stoping” and has become the predominant direction of mining with the advent of rock blasting and power drills. In particular, the technique commonly involves drilling multiple bores upwards from the cavity into the rock strata towards the ore deposit above. Explosive charges are then set in the bores to blast away the intervening rock and to access the ore deposit directly. Indeed, the bores and the explosive charges may extend into the ore deposit itself, which together with the intervening rock then collapses into the cavity below for removal.
A significant problem associated with this mining technique is associated with drill rod failure when drilling the multiple bores extending upwards into the rock strata towards the ore body. The individual bores drilled are often tens of metres long (for example, in the range of 20 to 60 metres) and the drill rods which extend over that length may only have a diameter of about 80 millimetres. As the composition and properties of the rock strata will typically vary through its depth, the drill rods are subjected to varying and also somewhat unpredictable loading during the drilling of each bore. Perhaps not surprisingly, therefore, the failure or breakage of a drill rod is not uncommon when the multiple bores are being drilled to lay the explosive charges. This has the problem that a section of drill rod, which may, for example, be fifteen or twenty metres long with a mass in the range of 100 kg to 500 kg, is left lodged in the bore extending upwards from the cavity. The pressure produced by the rock strata can alter either naturally, or as a result of further drilling of adjacent holes. This can result in the broken drill rod dislodging from the bore hole and falling into the cavity. It is therefore not difficult to imagine that the hazard posed to personnel and/or to equipment by such a massive broken drill rod section, which could unexpectedly drop out of the bore, is extreme.
In the event of such a drill rod failure in a bore extending above horizontal (where the risk of the broken drill rod section dropping out exists), occupational health and safety regulations in many countries require the affected bore to be covered and/or otherwise rendered safe before work in that particular area may continue. In the absence of a tailored solution to this problem to date, however, miners have had to improvise with very provisional and suboptimal measures. These have not only been extremely time-consuming, leading to long delays in the further progress of the mining, but the real safety provided by such provisional measures has at times also been questionable.